Friday, March 14, 2008

Variations on an Optical Theme

While the two major types of optical telescopes are the refractor and the reflector, it is also useful to be aware of the basic variations in reflector design, especially when you think about choosing a telescope for yourself (see the next chapter). We have already seen that the simplest reflector (prime focus) focuses its image at the front of the telescope, introducing the possibility that the observer may block the image. The Newtonian focus instrument, as mentioned, overcomes this problem by introducing a secondary mirror to direct the focus to an eyepiece at the upper side of the instrument. This remains a popular arrangement for small reflecting telescopes used by amateur astronomers. This arrangement is unwieldy, however, for a large telescope. Imagine trying to get to the “top” of a telescope 6 feet long, perched on a 6-foot pedestal.

Some larger reflecting telescopes employ a Cassegrain focus. The image from the primary mirror is reflected to a secondary mirror, which again reflects the light rays down through an aperture (hole) in the primary mirror to an eyepiece at the back of the telescope.

Finally, a coudé-focus (coudé is French for “bent”) reflector sends light rays from the primary mirror to a secondary mirror, much like a Cassegrain. However, instead of focusing the light behind the primary mirror, another mirror is employed to direct the light away from the telescope, through an aperture and into a separate room, called the coudé-focus room. Here astronomers can house special imaging equipment that might be too heavy or cumbersome to actually mount to the barrel of the telescope. Reflecting telescopes have their problems as well. The presence of a secondary mirror (or a detector, in the case of a prime-focus reflector) means that some fraction of the incoming light is necessarily blocked.

Although reflectors do not experience “chromatic aberration” (since light does not have to pass through glass), their spherical shape does introduce spherical aberration, light being focused at different distances when reflecting from a spherical mirror. If not corrected, this aberration will produce blurred images. One common solution to spherical aberration is to use a very thin “correcting” lens at the top of the telescope. This type of telescope, which we will discuss more in the next chapter, is called a Schmidt-Cassegrain, and is a popular design for high-end amateur telescopes.